Can't install extensions in /vendor/civicrm when using Drupal + path prefix language negotiation

In the following environment:

• Drupal
• Multi-lingual
• Using path prefix language negotiation (ex: https://example.org/fr/civicrm/dashboard?reset=1)

If we install CiviCRM extensions in /vendor/civicrm/, and those extensions load custom CSS files, the URL generated for those CSS files will include the language prefix as well.

If you look at CRM/Core/Config.php, _setUserFrameworkConfig :

if (defined('CIVICRM_UF_BASEURL')) {
$this->userFrameworkBaseURL = CRM_Utils_File::addTrailingSlash(CIVICRM_UF_BASEURL, '/');

//format url for language negotiation, CRM-7803
$this->userFrameworkBaseURL = CRM_Utils_System::languageNegotiationURL($this->userFrameworkBaseURL);
[...]

Then in CRM/Extension/System.php getFullContainer(), we see :

$containers['cmsvendor'] = new CRM_Extension_Container_Basic(
$vendorPath,
$this->parameters['userFrameworkBaseURL'] . DIRECTORY_SEPARATOR . 'vendor',
$this->getCache(),
'cmsvendor'
);

And so userFrameworkBaseURL gets used for the URL base for CSS.